1. Field of the Invention
The present invention relates to airflow control for a boiler or the like, and more particularly to an airflow control apparatus for use in a system where a ventilator is driven by a motor energized from a variable frequency power supply.
2. Description of the Prior Art
A conventional control apparatus of this type known heretofore has a constitution of FIG. 1, wherein there are shown a commercial power line 1, breakers 2 and 3, a motor 4, a ventilator 5 connected directly to the motor, a VF (variable frequency) power supply 6, a controller 8 for feeding an operating frequency signal to the VF power supply 6, and a signal line 7 of the controller 8. FIG. 2 graphically shows the relationship between an operating frequency and a frequency command signal outputted from the controller 8.
FIGS. 3 and 4 are explanatory diagrams relative to FIG. 1, wherein: t.sub.1 is a time when an increment command is outputted from the controller 8; t.sub.2 is a time when increasing the output of the VF power supply 6 is completed; t.sub.3 is a time when a fault occurs on the signal line 7; and t.sub.4 is a time when a change in the output of the VF power supply 6 is completed posterior to occurrence of the fault.
In such conventional control apparatus, the operation is performed in the following manner.
In FIG. 1, a voltage is applied from the commercial power line 1 via the breaker 2 to the VF power supply 6. The rotational speed n of the motor 4 is dependent on the output frequency F of the VF power supply 6 as given by Eq. (1) below. ##EQU1##
Thus, the rotational speed n varies in proportion to the frequency F of the applied voltage. It follows that a change in the output frequency F of the VF power supply 6 causes a change in the rotational speed of the motor 4. The ventilator 5 connected directly to the motor 4 sends to a boiler an output airflow Q which is substantially proportional to the rotational speed n of the motor 4. In an exemplary case of a power plant, when there arises a change in the demand from a power system or in a fuel feed condition, the airflow Q required for the boiler comes to vary as well, and a command signal for changing the airflow Q is outputted from the controller 8 via the signal line 7 to the VF power supply 6, which then produces an output of a frequency F corresponding to the required airflow Q.
FIG. 2 graphically shows the relationship between the operating frequency and the frequency command current outputted from the controller 8. It is plotted in this gragh that the controller 8 produces a frequency command signal of a current from 4 to 20 mA for varying the operating frequency of the VF power supply 6 in a range of 10 to 40 Hz.
FIG. 3 is an explanatory diagram relative to a normal operation performed in the conventional apparatus of FIG. 1. The controller 8 requests increase of the airflow Q at time t.sub.1 and feeds a frequency command signal via the signal line 7 to the VF power supply 6. Generally the VF power supply 6 is so designed as to increase or decrease the output thereof at a predetermined rate in response to the request for variation. Accordingly, the output frequency F of the VF power supply 6 is somewhat delayed with respect to the variation request time t.sub.1, and the action to execute the variation request is terminated at time t.sub.2. And the rotational speed n of the motor 4, hence the output airflow Q of the ventilator 5, comes to vary in accordance with the output frequency F of the VF power supply 6.
FIG. 4 shows an example with a fault or abnormal state caused on the signal line 7 of FIG. 1. In this case where the fault is disconnection of the signal line 7, the controller 8 keeps requesting a predetermined airflow Q with its output remaining unchanged, but the signal line is interrupted at time t.sub.3 so that no further input is fed to the VF power supply 6. As described previously in relation to FIG. 3, the output frequency F of the VF power supply 6 is decreased to its lower limit with some delay, and simultaneously the rotational speed n of the motor 4 is also decreased to bring about a reduction in the output airflow Q of the ventilator 6.
In the conventional apparatus having the constitution described above, when the frequency command signal from the controller 8 becomes abnormal or comes to be extinct due to disconnection or other fault of the signal line 7, the VF power supply 6 drives the motor 4 by the aforesaid fault signal. Consequently, some disadvantages have been unavoidable heretofore including that the ventilator 5 connected directly to the motor 4 fails to maintain the required airflow, thereby inducing the possibility of extinguishing the fire in the boiler or even causing its explosion.